There are several existing methods for measuring thickness of thin films, including spectroscopic ellipsometry, spectroscopic reflectometry, white light, or low-coherence interferometry, and thermal wave measurement. Each of these methods has limitations.
For example, spectroscopic ellipsometry is a widely-used and highly accurate measurement method for thin film thickness; however, it is a single-point measurement method and typically requires several seconds of measurement time per data point, making it much too slow for high-speed mapping as is required in an in-line system.
Spectroscopic reflectometry is also a widely-used and accurate measurement method for thin film thickness. Like spectroscopic ellipsometry, it is a single-point measurement with typical acquisition times of at least 1 second per data point. This method is not sufficiently fast for high-speed mapping. Commercial systems using spectroscopic reflectometry include those available from OceanOptics, Filmetrics and N&K, to name a few. U.S. Pat. No. 7,304,744 discloses a method employing spectroscopic reflectometry.
The white light, or low-coherence interferometry, technique takes advantage of the same physical phenomenon as spectroscopic reflectometry, but measures reflectance indirectly by means of a white light interferometer instead of a spectrometer. This technique can conceivably enable an imaging mode in which many points are measured simultaneously, but it is a technically difficult and expensive method. U.S. Pat. Nos. 7,468,799 and 7,483,147 relate to methods of measuring thin films using the white light, or low-coherence interferometry, technique.
The thermal wave technique can also be used to measure thermal characteristics of a sample and to draw conclusions on feature thickness such as film thickness. However, this technique is also a single point measurement and requires use of two lasers, one laser to create thermal waves, another to probe the thermal waves, thus it is slow, expensive and may require complicated compensation of thermal lens effects.
None of the aforementioned measurement methods has been adequate to make accurate in-line thickness measurements in the context of high volume SiOG production. Thus, there is a need in the art for new methods and apparatus for high-speed mapping of film thickness such as is required for accurate in-line measurement in high-volume production of SiOG (Silicon-on-Glass).